In the image forming apparatus such as a copying machine and a printer, an exposure is given based on image data, whereby, an electrostatic latent image is formed, and then, the electrostatic latent image is developed to form a toner image, and this toner image is transferred onto a sheet. Then, the sheet is conveyed to a fixing unit where the sheet is interposed by a fixing member composed of a heat roller and a pressure roller to be conveyed, whereby, the toner image is fixed on the sheet.
Incidentally, when a sheet jam is caused on a fixing unit or in the vicinity of the fixing unit in the image forming apparatus like this, there appears a situation where toner which is not fixed yet keeps adhering to the fixing member inside the fixing unit.
Therefore, toner adhering to the fixing member is removed by allowing a fresh sheet to pass through the fixing member and by transferring toner adhering to the fixing member onto the fresh sheet, through operations called a cleaning mode.
Unexamined Japanese Patent Application Publication No. 2003-270,985 (FIG. 1 on the first page) discloses a fixing unit in which a recording material is caused to pass through a nip portion formed by a fixing roller and a pressure member, and the fixing roller is rotated reversely after image forming for a prescribed number of sheets is completed, whereby, cleaning is carried out and toner contamination can be prevented.
Further, Unexamined Japanese Patent Application Publication No. 2006-154,192 (FIG. 1 on the first page) discloses an image forming apparatus in which cleaning is carried out surely and efficiently without intermingled post processing operations, by ejecting a sheet for cleaning that has passed through a fixing unit for cleaning after clearing jam and a sheet on which a toner image is formed to different ejecting portions.
Still further, in Unexamined Japanese Patent Application Publication No. 2002-287,446 (FIG. 1 on the first page) discloses image forming operations for the second sheet and thereafter for the first time after detection of completion of image forming on the first sheet when detecting completion of image forming by providing a sensor for confirming sheet ejection, for preventing occurrence of continuous jam for plural sheets in the case of operating again after clearance of sheet jam, and when operating again after jam clearance accompanying occurrence of sheet jam.
Still further, in Unexamined Japanese Patent Application Publication No. H9-292,803 (FIG. 1 on the first page), there is a description to prohibit printing operations of an apparatus and redoing of cleaning until the moment when the prescribed conditions are satisfied, after doing of cleaning, for the purpose of doing cleaning of a heat fixing unit rationally without any problem, in an image forming apparatus wherein a recording material on which a toner image is formed is brought to a nip portion formed by a heating member and a pressure member, to be interposed and conveyed for heat fixing.
Still further, in Unexamined Japanese Patent Application Publication No. 2004-126,426 (FIG. 1 on the first page), there is a description to clean toner or the like on a roller by conveying a cleaning medium to a fixing unit to be interposed by a nip portion in the course of cleaning, and then, by lowering the conveyance speed of the medium to be lower than the regular conveyance speed in an image forming mode under the condition that the cleaning medium is interposed by the nip portion, and thereby, by causing a fixing roller and a pressure roller to slip on the medium, for the purpose of removing dirt on a rotating body for fixing in a fixing unit simply and effectively, without relying on conventional complicated cleaning mechanism and heat and without requiring disassembly of a fixing unit.
Still further Unexamined Japanese Patent Application Publication 2006-39, 477 (FIG. 1 on the first page) discloses an image forming apparatus having a cleaning mode for repeating two or more times a control step in which a recording material is held in a fixing nip, driving is stopped, the fixing nip is heated and cooled. And then the recording material is conveyed. Said image forming apparatus has a pressing force alternation means for altering a pressing force, and also a pressing force determining means for determining the pressing state. Heating temperature/time, cooling temperature, or an amount of step conveyance in the cooling mode is altered according to the pressing state.
Namely, various types of methods for doing a cleaning mode effectively are described in the respective Patent Document stated above.
However, the inventor of the present invention found out, after keen investigations, that conditions for sheets to be used for cleaning to cause a jam easily still are in existence when doing the cleaning mode of this kind, and a jam is further caused even in the case of cleaning, when sheets which cause a jam easily are used for leaning, to stop an apparatus, resulting in a decline of productivity of an image forming apparatus.